Infrared laser absorption spectroscopy is an effective tool for detecting trace gases. However, the usefulness of laser spectroscopy is limited by the lack of broadly tunable mode-hop-free sources, for example in the mid-infrared (IR) region, which may be defined as wavelengths between about three and about 30 micrometers (μm). A mode-hop-free source may be defined as a source that exhibits laser emissions over a continuous range of wavelengths or frequencies without abrupt changes in the laser output power or phase.
Quantum Cascade (QC) and Interband Cascade (IC) lasers are suitable light sources for spectroscopic applications. The high power of QC and IC lasers permits the use of advanced detection techniques that improve the Signal-to-Noise (S/N) ratio of trace gas spectra measurements and decrease the apparatus size. In addition, the large wavelength coverage available with QC and IC lasers allows monitoring numerous molecular trace gas species.
Spectroscopic applications require single mode operation, which can be achieved by introducing a distributed feedback (DFB) structure into the QC Laser (QCL) active region. Experiments using distributed feedback lasers have demonstrated the efficacy of these devices for sensitive and highly selective real time trace gas concentration measurements based on absorption spectroscopy, with sensitivities of several parts per billion. An example of such studies is presented by K. Namjou et al. in Optics Letters, V. 23, n. 3, published Feb. 1, 1998 and entitled “Sensitive absorption spectroscopy with a room-temperature distributed-feedback quantum-cascade laser,” which is incorporated herein by reference as if reproduced in its entirety.
Although DFB QCLs show high performance and reliability, they are useful only over narrow wavelength ranges. This is because the range of wavelength tuning of the emitted laser radiation is limited by the tuning range of the DFB structures. Typically, the maximum tuning range of DFB QCLs is of about ten inverse centimeters (cm−1), which is achieved by varying either the temperature of the gain chip or cavity or the laser injection current. One of the disadvantages of thermal tuning is that it changes the effective gain of the QCL, which causes the output laser power to decrease with increasing temperature of the QCL chip.
To take full advantage of the wavelength tunability potential of a QCL, an external cavity (EC) configuration can be applied. However, high quality or effective anti-reflective (AR) coatings, e.g., that have low reflection, low absorption, and high transmission at a continuous range of wavelengths, are necessary for mode-hop-free EC laser operations and are not generally available for the mid-IR spectrum. The lack of effective AR coatings in the mid-IR range makes it difficult to achieve laser wavelength tuning without experiencing mode-hopping, e.g., without discontinuities or gaps in the wavelength or frequency range of the laser output. A laser that exhibits mode-hopping is not useful in high resolution spectroscopic applications, such as spectral measurements of rotational-vibrational molecular transitions, where information is needed over a continuous wavelength range of measurements.